This is something i have also encountered on R81.10.15. We encountered another bug with cluster of 1800, when the cluter WAN IP stopped responding and all associated VPN tunels (sourced from VIP) also stopped working. R&D was not able to help me as I was not able to replicate the issue on call with them. We tried testing on borrowed Spark 2000 with trial licences for testing purposes but the issue repeated. Then I have tried upgrading the cluster to R81.10.15 and I am seeing same issue as OP. Web server error and this line in /var/log/messages
2025 Feb 11 11:24:27 xxx user.err root: [System error] CODE_SYS_ERROR (8712.149.48) - sys/OSTLORM.lua:48: Failed to connect (Log reference: 1739269467)
P.S.: We are using local management and cluster is not connected to cloud Spark Management